In the semiconductor industry, aluminum and aluminum alloys have been used as the traditional interconnect metallurgies. While aluminum-based metallurgies have been the material of choice for use as metal interconnects over the past years, concern now exists as to whether aluminum will meet the demands required as circuit density and speeds for semiconductor devices increase. Because of these growing concerns, other materials have been investigated as possible replacements for aluminum-based metallurgies. One highly advantageous material now being considered as a potential replacement for aluminum metallurgies is copper. This is because copper exhibits a lower susceptibility to electromigration failure as compared to aluminum as well as a lower resistivity.
Despite these advantages, copper readily diffuses into the surrounding dielectric material during subsequent processing steps. To inhibit the diffusion of copper, copper interconnects are often times capped with a protective barrier layer. One method of capping involves the use of a conductive barrier layer of tantalum or titanium, in pure or alloy form, along the sidewalls and bottom of the copper interconnection. To cap the upper surface of the copper interconnection, a dielectric material such as silicon nitride, Si.sub.3 N.sub.4, is typically employed.
Due to the need for low temperature processing after copper deposition, the silicon nitride layer is deposited at temperatures below 450.degree. C. Accordingly, silicon nitride deposition is typically performed using plasma enhanced chemical vapor deposition (PECVD) or high density plasma chemical vapor deposition (HDPCVD) wherein the deposition temperature generally ranges from about 200.degree. to about 500.degree. C. PECVD and HDPCVD silicon nitride has been used for many other applications in semiconductor device manufacturing. However, in using a silicon nitride cap for copper interconnects, conventional PECVD or HDPCVD silicon nitride creates reliability problems. In particular, silicon nitride films deposited using conventional PECVD or HDPCVD processes generally exhibit poor adhesion to the copper surface. For instance, some nitride films delaminate and form blisters over patterned copper lines, particularly during subsequent dielectric depositions, metallization, and chemical-mechanical polishing.
These results are indicative of how the silicon nitride film might adhere to the copper in actual fabrication processes. After being deposited onto copper metallurgy, additional insulating layers generally will be deposited over the silicon nitride film. However, subsequent deposition of insulating layers onto the nitride film will produce stress which can cause the silicon nitride film to peel from the copper surface. This delamination results in several catastrophic failure mechanisms including: lifting intermetal dielectrics, lifting copper lines, and copper diffusion from uncapped copper lines. Such results are generally seen in dual damascene processing wherein delamination of the silicon nitride RIE stop layer generally occurs during copper chemical mechanical polishing (CMP).
Prior art nitride to copper adhesion requires siliciding the copper surface by reacting it with silicon. This prior art method has two drawbacks: increases the copper sheet resistance due to silicon reacting with copper and diffusion therein; and marginal nitride to copper adhesion due to incomplete or partial copper silicide formation.
In view of the drawbacks mentioned with prior art copper interconnect structures, there is a continued need to develop a new process of facilitating the adhesion of an inorganic barrier film to copper surfaces which are present on interconnect semiconductor structures.